Method of producing cellulosic pulp



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wJm bmOm mDOwZwGOm-MPMI ATTORNEY Nov. 17, 1959 D. s. CUSI METHOD oF PRODUCING CELLULOSIC PULP 5 Sheets-Sheet 5 Filed June 14. 1954 whom 1mm ATTORNEY 2,913,362 METHOD F PRODUCING CELLULOSIC PULP Dante S. Cusi, Mexico City, Mexico, assignor to International Pulp Products, Inc., Washington, D.C., a corporation of Panama Application June 14, 1954, Serial No. 436,637

4 Claims. (Cl. 16255) This invention relates to a novel method of treating vegetable materials of generally non-homogeneous structure so as to produce valuable cellulosic products which are generally fibrous pulps, but may also include nonbrous products or fibers of different or non-homogeneous physical or chemical properties. It relates further to the preparation of a plurality of such products, each product being substantially homogeneous in character. The invention relates more particularly to a novel method and corresponding novel products, together with variations thereof, of producing from non-woody vegetable materials containing a plurality of types of cellulosic structural elements a plurality of substantially homogeneous cellulosic products, each corresponding generally to one of said types of said constituent structural elements.

Generally speaking, fibrous vegetable materials are Aabundant throughout the world, and most are fundamentally potential sources of cellulose and cellulose products. Cellulose products such as dissolving pulps, papermaking pulps, and related materials are in increasing demand throughout the world, yet of all the vegetable source materials, only wood has been used to any great extent. This is true even though wood is frequently locally in severe short supply and must be transported over great distances.

Other vegetable materials, as abundant or Amore so than wood, such as sugar cane bagasse, cotton stalks, corn stalks, fiax, ramie, kenaf, many cacti such as the magueys,vthe pineapple, and other agricultural straws, stalks and stemshave not been utilized as sources of cellulose products even though they have a high fiber and cellulose content. One of the principal reasons is that these materials lack the uniformity of structure which characterizes wood. This non-uniformity, or heterogeneity heretofore has hindered greatly the chemical and mechanical treatments used to liberate fibers or otherwise transform the crude materials into industrially usable products. As a consequence, the intense demand for a wide variety of cellulose and fiber products has been supplied from wood. This has brought about a diminishing supply, growing transportation problems and increasing cost.

The variety of cellulose and fiber products required in industry is great, ranging from high alpha or dissolving pulps for the chemical industry to crude mechanical pulps for the structural board industry. It includes the well known-papermaking pulps designed for such specific ultimate uses as newsprint, magazine book papers, Wrapping papers, corrugating board, and the like.

All these uses require special properties which must be engineered into wood pulp by a wide variety of pulping y methods and pulp treatments.

The present invention includes methods of pulping and fiber treatment whereby advantage is taken of the unique heterogeneous character of the non-Woody vegetable fiber materials. As will be seen, these methods afford direct preparation of a number of valuable cellulose and fiber products.

In these methods the very heterogeneity of United States Patent ICC the vegetable materials, heretofore a serious limitation to their utilization, contributes critically to the technologie success of the invention. Thus, from one single vegetable material such as sugar cane bagasse, for example, several separate pulps, each of significant value and high quality are produced, where by prior methods only one pulp of relatively low quality is obtained.

In contrast to the relatively homogeneous structure of wood commonly used for pulping, the vegetable materials enumerated previously characteristically contain nonfiber cellulose constituents as well as several different constituent fiber types. These different fiber types characteristically possess substantially different physical and chemical properties. Consequently they respond differently to any given set of pulping conditions. In other words, they possess different pulpabilities. Thus,

bagasse for example, consists of parenchyma or pith, and

with a good content of pure cellulose, closely bonded together in dense bundles. These tissues are relatively refractory or resistant to pulping, i.e., have a low pulpability. The second 'constituent tissue, the vascular canals, is composed of monocellular fibers grossly similar in appearance to the peripheral fibers, but because of its structural and chemical characteristics is readily penetrated and attacked by pulping chemicals, and hence has a high pulpability.

The term pulpability yas used herein designates the relative ease with which a tissue or fiber constituent responds to the action of pulping, whether said action is purely chemical, a consequence of application of heat or mechanical action, or a combination of any of them. The problem of varying pulpability is peculiar only to heterogeneous materials, since relatively homogeneous materials such as wood consist of fibers which are all of nearly equal pulpability. However, the art is familiar with the fact that different kinds of fiber materials require different pulping conditions due to their differing pulpabilities, though heretofore the problem of differing pulpabilities of fibers existing in one and the same vegetable material does not appear'to have been adequately dealt with. The pulpabilityof fibrous vegetable materials is determined by the physical and chemical character of said material as well as the physical and chemical ch-aracter of the constituent fibers themselves, i.e., the amount of lignin, hemicelluloses, gums, pectins, etc., the degree of polymerization, the thickness of the fiber walls, the thickness and density of the fiber bundles, the interspersion of vascular canals between fibers, the characteristic tissue structure prescribed by the position, bonding and other characteristics of one fiber relative to the others. These properties govern mainly the ease of penetration and attack by pulping or softening chemicals.

This invention is based partly upon the discovery that one of the most important factors'contributing to the low quality of prior papermaking pulps from such vegetable materials as heretofore mentioned is the presence in the finished pulps of these differing typesl of vconstituent fibers, typified by the two types noted in pulping processes to recover all recoverable fiber into such a heterogeneous pulp.

The invention is based upon the further discovery that certain novel pulping methods are capable of separating the fibrous elements of a heterogeneous vegetable material into what may be termed fractions of substantially homogeneous pulps. These fractions are defined by their characteristic pulpabilities as determined by the predominating character of the constituent fiber types. This affords a maximum realization of the valuable special properties of each constituent fiber type, and leads to many economic and technologie advantages, as will be readily seen from the following more detailed description. In many instances, the invention makes possible the recovery of valuable fiber products from vegetable materials that have heretofore been worthless.

Sugar cane bagasse is a typical example of a heterogeneous vegetable fibrous material. It is the crushed and macerated cane remaining after expression of the sugar juice from sugar cane. It has been utilized heretofore to make various paper and board products. Ordinarily it is depithed before pulping, and several methods are known for doing this. The paper products are then made from the depithed fiber employing pulping methods designed to produce a single pulp. This prior type of pulp oers a striking example of how different constituent fiber types impair the quality and value of a nonhomogeneous pulp. These prior pulps, particularly the semi-chemical ones, are reduced in quality owing to the presence of large proportions of hemicelluloses. A1- though it is technically possible to remove the latter by hydrolysis, thus to recover the high alphacellulose constituent fibers, the cost of doing so is prohibitive, because of the low yield and because the cellulose obtained by said hydrolysis is itself attacked by hydrolysis. This means lower pulp strength and lower quality. Moreover, in doing so, that portion of the fiber high in hemicelluloses would be lost, with a consonant high consumption of chemicals.

In the foregoing the production of two fibrous pulps has been discussed, but it is to be understoodthat the invention may be utilized to produce a greater number, each characterized by substantial homogeneity of physical and chemical properties. lt may be utilized to obtain substantially homogeneous pulps from many vegetable materials which cannot be pulped by prior methods, even though they possess considerable proportions of desirable fibers. Leafy plants containing different types of fibers in the leaves, the stems, and the trunk, may be treated in accordance with the invention by virtue of the remarkable flexibility of operation and control over the character of the pulp fractions aforded by the present invention. This invention may be utilized to obtain substantially homogeneous pulps from a heterogeneous mixture of woods of different origin, when, because of special climatic conditions such as found in the tropics, many species grow together, and it would be impractical or too costly to pulp separately the wood of each species.

The invention is based upon the principle of preferential or selective pulping, conducted in such a manner that the constituent fiber types of appreciably different pulpabilities are successively softened and recovered as a separate pulp fraction from the unpulped constituents. It will be described in detail as it relates to the treatment of bagasse and cereal straw.

The gist of the invention is the practical utilization of the discoveries heretofore mentioned. Broadly stated, it comprises the application of conditions such as chemical environment, mechanical treatment, time and temperature in such a way that the extent of pulping is controlled so as to accomplish distinct stages of successive pulping of the constituent fiber types according to their pulpablities This may be accomplished by applying in a primary digestion sutliciently mild pulping conditions that the constituents of highest pulpability alone respond. The pulped material is then separated, and the remaining unpulped constituents subjected to a secondary digestion. Fractionation and sclective pulping may be reapplied in a still further stage if the pulp from the secondary stage leaves a residual pulpable material. In this way the heterogeneous starting material is converted into a series of substantially homogeneous pulps. With a given chemical environment, the extent of pulping may be controlled by mechanical action, time and temperature, each of which is a factor governing the rate of pulping. Practically, it is advantageous to employ a combination of control over degree of pulping and rate of pulping. For example, a relatively mild pulping chemical environment is selected in order to soften the most highly pulpable constituent fiber type. The selective action may be considerably accentuated by employing vigorous agitation to hasten penetration of the chemical.

Inasmuch as various means of controlling the extent of pulping are well known in the art, and the specific pulpabilitycharacteristic of each constituent fiber type of any heterogeneous vegetable fiber material is readily determinable by those skilled in the art, it is to be und erstood that this invention is not limited to any specie manner of controlling the individual pulping operation. It has been found, however, that certain specific conditions of operation are especially advantageous in the practice of this invention. If observed, these conditions will assist materially in establishing satisfactory -control over the extent of pulping. They are enumerated below.

(1) Relatively mild primary chemical cooking environment to accentuate the difference in pulping degree between the constituent fiber types.

(2) Relatively low temperature in primary cooking to accentuate the difference in pulping time between the constituent ber types.

(3) Selectively vigorous mechanical action to accelerate the penetration of liquids throughout the heterogeneous mixture, affording rapid contact between all liber elements and the chemical pulping liquors.

To the above conditions will be added intermediate mechanical defibering of pulp (if necessary) and separation steps as the fiber types are successively pulped.

Employing the foregoing fundamental conditions, it has been found that successive pulps are produced readily, each particularly valuable by virtue of outstanding physical and chemical homogeneity.

The terms mild, mild pulping, and mild pulping conditions as used .in this specification will be readily understood by those skilled in the art of pulpmaking to refer to definite conditions known and recognized by them t0 produce a low degree of attack upon vegetable fibrous material. A chemical reaction such as the one used in pulping, which has the purpose of attacking and solubilizing or destroying an element of a vegetable material, can obviously be mild or strong depending upon the degree of action. Generally the degree of action depends on several factors such as temperature, time and concentration of chemicals. The lower the temperature and concentration, the shorter the time, the milder will be the action on the vegetable matter. Conversely, the opposite conditions result in a stronger digestion. A very important factor, superimposed upon the aforementioned conditions is the kind of chemical used for the pulping. Some pulping chemicals used in industry are known to be classified as mild because of their inherent action, such as for instance the neutral sulte chemicals, whereas some are strong, such as caustic soda (NaOH).

Mildness or strength of pulping conditions depend therefore on the following factors:

Time, Concentration of chemicals,

Kind of chemicals,

Temperature. v

It is within the skill of the art, for the pulpmaker to adjust, regulate, and/or select these factors to achieve his desired pulping effect.. In this invention, the ternperature factor has been treated separately because of its importance in accentuating the difference in pulping time for the constituent fiber types. v

In the exemplary case of sugar cane constituent fibers, as previously described in detail, the peripheral or outer shell fiber is relatively resistant to attack under mild chemical environment and low cooking temperature, i.e., mild pulping conditions, and is likewise relatively more resistant to penetration than the vascular canal fibers, even under vigorous mechanical agitation. Moreover, the available chemical is rapidly consumed by the constituents having a high pulpability, and very little or none is left to act upon the more resistant, low pulpable, constituents. l The pulping process hence reaches successive stages that are relatively widely separated as to time. As these successive stages are reached, the cooked mixture of fibers may be subjected to defibering and subsequent screening to recover the first pulp product. At this stage the peripheral fiber material is substantially intact, yet the less resistant vascular canal fibers are softened sufficiently for defibering. When the cooking or digestion is carried out by the well known mechanochemical method, a separate defibering step is usually unnecessary, inasmuch as the defibering takes place simultaneously with the pulping or softening. Separate defibering, if necessary, is carried out according to conventional methods using conventional defibering equipment. The debered product, a mixture of hard unattacked fiber particles and defibered pulp, may be separated by screening. The screen may be of any of the conventional types such as a cylindrical screen, a centrifugal screen, or a' fiat screen, which may or may not be of the vibrating or pulsating kind. For best capacity per unit of installation, the centrifugal or vibrating fiat type of screen is preferred.

The pulp fraction which passes through the screen at the first stage of the process may be designated Frac' tion A pulp. It is a novel pulp, consisting almost entirely of that constituent fiber type of highest pulpability. This first or Fraction A pulp is composed of those parts of the starting material which contain a greater proportion of hemicelluloses. In the specific case of bagasse it consists essentially of the shorter vascular fibers, and also contains the residue of the parenchyma or pith not removed in the depithing operation.

A distinguishing characteristic of Fraction A pulps is their relative slowness and high density. This property contributes to the reduced quality of prior heterogeneous pulps, and is caused principally by its characteristic properties, i.e. general shortness ofiibers, their very small diameter, and their thin cellular walls; and their high content of hemicelluloses which have a marked tendency to swell in water. Fraction A pulps are hence inherently highly refined or hydrated types of pulps. They are therefore very easy to beat. They produce dense sheets of high rigidity, high rattle, and exceptionally high tensile and Mullen strength, very good formation and low tear resistance. These properties are frequently desired, and can be obtained to some degree from conventional wood pulps, but only by prolonged and vigorous .beating and refining. The papers made from Fraction A pulp are extremely dense and are valuable Where rigidity, lack of porosity, and good susceptibility to supercalendering is required. Papers having a high crush resistance, grease resistant papers, parchment, or glassine may be made from these pulps, either alone or mixed with varying proportions of ordinary wood pulp or other pulp. Their industrial importance will be apparent to those skilled in the art. Large tonnages of pulps having these properties are demanded by the paper industry annually.

In addition to direct use in making specialty papers, our novel Fraction A pulps, particularly those from such materials as bagasse, areI valuable for blending with many other pulps, such as sultite, sulfate, or mechanical wood pulps, for the purpose of imparting high tensile strength. The net effect of blending minor proportions of Fraction A pulps is practically equivalent to subjecting the sulfite or sulfate pulp to technologically difficult and expensive Y refining operations. Fraction A pulp as a blending pulp hence affords multiple advantages, economic'and technological, and affords high quality paper 'products at lower cost, reduced power consumption'and minimized equipment and overhead costs. The high quality of such blended paper stocks is a result of the near absolute control over physical properties made possible by the use of exact and predetermined proportions of homogeneous uniform blending agentsa kind of control which is extremely diflicult to attain by beating and/or refining. Moreover, the blend provides a paper of remarkably smooth surface and excellent formation.

The unpulped material rejected in the screening operation is likewise a novel material, distinguished both by its chemical and physical character. It is composed practically enitrely of thelarger and stronger constituent fibers of the starting material. It is higher in lignin, lower in hemicelluloses, hasvrelatively low chemical reactivity and is high in pure cellulose. This material may be pulped Iby any known pulping process, in accordance with the kind of pulp desired. For example, mild chemical conditions, low cooking temperatures, and vigorous mechanical action will produce a semi-chemical pulp; or relatively stronger chemical conditions will produce a chemical pulp high in pure cellulose. Generally the final homogeneous pulp in the novel successive series of pulps has superior overall fiber properties compared with the pulps preceding it in the series. This is particularly true in thecase of bagasse. Its high pure cellulose content and long fibers make it comparable in every respect to the best wood pulp (or cellulose). It is superior to wood pulps in softness owing to the extreme thinness of the individual fibers. This affords surprisingly high folding endurance in the papers made from it, superior to any wood-fiber paper. Papers made from it also exhibit excellentformation and good tearing resistance.

In contrast to the case of bagasse, there are some plants l such as liax, ramie, and kenaf, which produce a series of pulps in accordance with this invention in which the first pulp obtained in the succession is of extremely high quality and composed of superior fibers, whereas-the 'succeeding pulps tend to be relatively high in lignin.

It will be understood by those skilled in the art that the process of this invention lends itself readily to continuous operation whereinv the various process steps are conducted successively in a series or line of equipment provided with means for recycling the various streams of fiber suspensions. The initial cooking or digestion employing mild conditions as'heretofore mentioned may be carried out as follows: A mechanical pulping device may be used which subjects the material to submerged impact blows, such as the hydrapulper. This device is charged with the starting material, such as depithed bagasse. Water and chemicals are also charged to give a 5% to 10% alkali cook (based on said starting material) at a consistency of 5% to 15%. The temperature of cooking may be from C. to boiling, and the pulping time about25 minutes to one hour.

The end of the first stage is readily determinable by gross examination of the cooked material, and the cooking is stopped when the first fiber constituent type is pulped. The device isrthen ldischarged to a screen for recovering theFraction A pulp, and the screen rejects charged to a second digesting stage. The first digesting stage, if carried out without vigorous mechanical action Awillgive acooked product in which 'thesoftened Frac- 7 tion A fiber must be detibered. The debered product is then Screened as above to recover FractionA pulp.

The character of treatment carried out on the residual unpu'lped material rejected from Fraction A will depend upon a variety of factors. If further homogeneous pulps are desired, and the residual material is heterogeneous, the material may be again pulped selectively as above, employing the same or less mild conditions to produce a Fraction B pulp and an unpulped residue. This latter residue which normally will be homogeneous may be pulped as desired. The number of pulp fractions in the Series of products is limited only by the heterogeneity of the starting material. Likewise the variety of treatments accorded the residual unpulped material rejected in the screening steps is of wide choice. The accompanying owsheets illustrate dagrammatically several overall processes. It will be understood that further conventional pulp treatments may be provided at appropriate stages in the process, such as Washing, neutralizing, bleaching, and the like.

Referring to Fig. l, the starting material, such as depithed bagasse, is given a mild digestion with vigorous agitation until the constituent fiber type having the highest pulpability is converted into a defibered pulp by the combined action of the pulping chemicals and the mechanical action. The device may be one as previously mentioned, operating at near the boiling point. The pulping chemical may be any of a wide variety of known pulping agents. Alkaline, neutral, or acid agents may beused, the selection being dependent mainly upon the type Aof pulp or cellulose product desired. Selection of specific pulping chemicals is within the skill of the art, but the amount used, i.e., the strength of the cooking liquor must in every case be mild enough for differential pulping under the physical conditions employed.

The digested materialis then led over a Vibrating hat pulp screen to separate the Fraction A pulp. The material rejected by the pulp screen is then subjected to a second digest-ion followed by a mechanical refining of the digested material. The conditions of the second digestion in the system of Fig. l are so controlled that the fiber is softened but not completely converted into a pulp. The cooking and subsequent mechanical refining, in this case, is for the purpose of producing a semi-chemical pulp as Fraction B, obtained by operating a pulp screen in circuit with the refiner. The screen rejects may be recycled directly back to the'refiner, or may be recycled to the second digestion step, as shown in the drawing. Also the recycle stream may be split, part going to the refiner and parts to the digester. Manipulation of the recycle streams offers surprisingly close control over the properties of Fraction B.

In Fig. 2, the starting material is given a mild digestion or cook as in the system of Fig. 1. The yprimary cooking is carried out to the stage of softened fibers of the constituent fiber type having the highest pulpability, as Iin Fig. l, but in this case defibering does Ynot occur. The cook is then debered as by passing through a defibrator such as a coarse set Bauer disc mill or a Jordan engine or the like. Alternatively the digestion may be carried out to a stage of more o r less complete defibering in the cooking apparatus (by `the application of simultaneous mechanical agitation), thus avoiding the necessity for a separate detibering step. Fraction A pulp is then separated by screening with a vibrating at screen. The material rejected by 'the screen, consisting of unpulped fibrous particles may be converted into a semi-chemical 'pulp by mechanical refining in a fine set disc mill operating Vin closed circuit with a screen for separating the semi-chemical pulp (Fraction B), -'and recycling the screen rejects. Instead of complete closed circuit operation, a portion of the screen rejects may be returned to the digestion step.

Referring to Fig. 3, an intermediate fraction may be 8 obtained from the initial digestion stage by screening out an initial Fraction A1. Here the screen reject is defbered as in Fig. 2, whereby a Fraction A2 is obtained. The screen rejects are converted into the semi-chemical pulp, as before.

Figs. 4 and 5 show modifications of the process which possesses certain apparent economic advantages. Here the initial digestion step is stopped short of complete softening of the first fiber constituent type, i.e., in the case of bagasse, the vascular canal constituent. It is followed by a mechanical refining step because satisfactory delbering requires a more intense mechanical treatment. Even in this instance, a homogeneous Fraction A pulp may be separated from the refined material by screening. The screen rejects are again digested and the digested material given a second refining and screening as shown in Fig. 4, whereby the Fraction B pulp is recovered. The rejected material may be recycled to the refner or to the digester somewhat as in the system of Fig. l. Instead of directly refining the second digestion, a defibering and screening stage may be inserted to recover a Fraction B, pulp, and the screen rejects from this step then refined. This modification is shown in Fig. 5.

The process of Fig. 6 is somewhat similar to that shown in Fig. 4. It differs in that a preliminary screening follows the first digestion to separate Fraction A1 pulp, thus relieving the load on the first refiner.

Alternative to the system shown in Fig. 6, as a variation thereof, the second rening step may be omitted. In this instance, the product of the second digester is screened directly to recover Fraction B pulp. The screen reject material is then recycled back to the second di gester, or it may go back to the first refining step.

Referring to Fig. 7, the starting material such as depithed bagasse is given a mild digestion with vigorous agitation until the constituent fiber type having the highest pulpability is converted into a pulp, by the combined action of the pulping chemicals and the mechanical action. The device may be one as previously mentioned, operating at near boiling point. The pulping chemicals may be any of a wide variety of known pulping agents. In this case the first or primary processes are exactly the same as shown in Fig. l. Fraction A pulp is recovered by screening, and the rejected material is then subjected to a secondary digestion with much stronger pulping conditions, such as higher temperature, higher pressure, and/or greater concentration of chemicals; the conditions being chosen so as to produce a chemical pulp in accordance with ordinary practice. The secondary digestion is followed by blowing into a blow tank, screening and washing-in.

In Fig. 8 another modification is shown similar to that of Fig. 7, the only difference being that a mechanical detbrator is employed after the primary cook.

It should be noted that it is very easy by the present invention to control rather closely the amounts of pulp that is obtained in each step. For instance, in the first step, the amount of A pulp obtained increases with increasingly strong pulping conditions. It decreases as the primary pulping conditions selected are milder. This type of process step control offers an excellent means of controlling the amount and quality of either fraction, for they are closely interrelated, the sharpness of one being determined to a large extent by the method of producing the other.

pulper, in accordance with the mechanochemical process for producing pulp.

homogeneous and of very good quality.

The vessel had a capacity of l36 cubic metersfand was equipped with a 250 H.P. motor. A typical charge was about 4000 kg. of (dry weight) depithed bagasse, 320 kg. (8% of the bagasse) of sodium hydroxide (NaOH), and boiling water to fill the vessel to its capacity. The proportion of bagasse to liquid may be as high as one part bagasse to eight parts of liquid, making the initial consistency about 11%.

The vessel was charged with the agitator in motion, the operation taking about 8 to 10 minutes. Some steam was injected to raise the temperature of the mixture as near to the boiling point as possible. Generally, after the boiling temperature is reached, the steam can be shut off, for the heat generated by the motion of the agitator is enough to maintain the temperature. The agitator was kept in motion for 30 to 40 minutes, and then the bottom valve was opened, without stopping the agitator, and the charge was dumped in a chest. The vessel was then ready to receive a succeeding batch.

Primary defbering From the chest the charge was pumped to a double disc Bauer mill, equipped with two 200 H.P. motors, running at 1450 r.p.rn. in opposite directions. The discs, which had an external diameter of 36 inches, were set at .045 in. The defibraton took about 40-45 minutes.

The defibered material was screened through a primary set of screens consisting of a at high frequency screen with 0.25 in. holes. The accepted stock was rescreened through two centrifugal screens in series with 0.125 in. and 0.045 in. round holes, respectively. The final accepted stock, after washing, weighed approximaterly 1500 kg. It was the Fraction A pulp.

Secondary digestion The rejected material from the screen was charged in a digester with a solution of NaOH containing 305 kg. of caustic (about 14% based on the dry weight of the material). The charge was recooked under autogenous pressure at 150 C. for two hours. The 4digester was then emptied through a blow tank, and the stock screened and washed in the ordinary manner. The material rejected by the screen was recycled to the digester. This final pulp weighed approximately 1300 kg., dry basis, and was Fraction B.

The foregoing example is shown graphically in Fig. 8 of the drawings.

EXAMPLE II The procedure of Example I was repeated, using cereal straw instead of depithed bagasse. essentially the same, and the yield of usable pulp was approximately the same. In the case of straw, however, a slightly greater amount of alkali was used in the secondary digestion (approximately 15-16%). Furthermore, the rejected material from the screening step of the secondary digestion should not be recycled, inasmuch as it consists of a small amount of nodes, rachises, and the like. The Fraction B pulp was remarkably Its characteristics indicate a wide use for it in the paper and related industries.

From the foregoing examples and from the flow diagrams shown in the drawings, it will be apparent that the present invention is remarkably flexible. It is capable of producing a wide variety of chemically and physically homogeneous pulp from one single heterogeneous starting material. Furthermore, these pulp fractions may be very closely controlled as to degree of pulping or refining, merely by varying the combination and sequence of steps. This liexibility of operation and facility of control is owing to the novel method of producing successive homogeneous pulps, and is one of the many practical advantages of the invention.

The particular combinations and sequence and num- The operations were y ber of process stages as illustrated in the flow sheets will depend to a large extent upon economic considerations. It should be noted,y however, that as the number of successive stages increases, the number, quality, value, and usefulness of the various fractions likewise tends to increase.

The fundamental advantage, however, will be realized merely by the preparation and recovery of Fraction A, as heretofore discussed. This action not only provides a novel valuable product, but improves significantly the quality and value of the residual material.

I claim:

1. The method of producing from sugar cane a substantially homogeneous pulp consisting principally of digested vascular fibre comprising separating from the crushed cane substantially its entire content of parenchymatous tissue, while leaving a residue composed of a heterogeneous mixture of substantially the original quantities of fibre-vascular bundles and substantially the original quantities of peripheral sclerenchyma, forming the said residue into a suspension in liquid of 5% to 15% solids, digesting the said suspension under predetermined elevated temperatures with an alkali cook to effect pulping of substantially all the fibres belonging to the fibrovascular bundles without substantial degradation thereof, while softening the sclerenchyma of the peripheral fibre bundles, and thereafter removing the peripheral sclerenchyma from the pulped vascular canal fibres.

2. The method of producing from sugar cane a substantially homogeneous pulp consisting principally of digested vascular fibre comprising separating from the crushed cane substantially its entire content of parenchymatous tissue while leaving a residue composed of a heterogeneous mixture of substantially the original quantities of fibre-vascular bundles and substantiallythe original quantities of peripheral sclerenchyma, forming the said residue into a suspension in liquid of 5% to 15% solids, digesting the said suspension under predetermined elevated temperatures to effect pulping of substantially all the fibres belonging to the fibre-vascular bundles without substantial degradation thereof, while softening the sclerenchyma of the peripheral fibre bundles, and thereafter removing the peripheral sclerenchyma from the pulped vascular canal fibres.

3. The method of producing from sugar cane a substantially homogeneous pulp consisting principally of digested vascular fibre comprising mechanically separating from the crushed cane substantially its entire content of parenchymatous tissue, while leaving a residue composed of a heterogeneous mixture of substantially the original quantities of fibro-vascular bundles and substantially the original quantities of peripheral sclerenchyma, forming the said residue into a suspension in liquid of 5% to 15% solids, digesting the said suspension under predetermined elevated temperatures to effect pulping of substantially all the fibres belonging to the fibre-vascular bundles without substantial degradation thereof, while softening the sclerenchyma of the peripheral fibre bundles, and thereafter removing the peripheral sclerenchyma from the pulped vascular canal fibres.

4. The method of producing from sugar cane a plurality of substantially homogeneous pulps comprising separating from the crushed cane substantially its entire content of parenchymatous tissue, while leaving a residue composed of a heterogeneous mixture of substantially the original quantities of fibre-vascular bundles and substantially the original quantities of peripheral sclerenchyma, forming the said residue into a suspension in liquid of 5% to 15% solids, digesting the said suspension under predetermined elevated temperatures to effect pulping of substantially all the fibres belonging to the fibre-vascular bundles without substantial degradation thereof, while softening the sclerenchyma of the peripheral fibre bundles, removing the peripheral sclerenchyma from the pulped vascular canal fibres, and thereafter digesting the ate'r'al so fremoved to .form a separate, pulp of the V858,411 peripheral s'clerenchyma. 930,874

1,153,934 References Cited in the le of this patent 1,311,864 UNITED STATES PATENTS 5 1.813,184

310,753 Walker Ian. 13, 1885 731,290 Drewsen June 16, 1903 12 McFarland Iuly 2, 1907 .Muller Aug. 10, 1909 -Lee .4.- Sept. .21, 1915 Richter June 30, 1931 McQuiston July 7, 1931 Darling Oct. 20, 1931 Connolly May 18, 1937 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pai-,ent N0 2,913,362' November 17, 1959' Dante .S Cusi Column 6 linear 24, for "eniely" read a-f entirely 'sh-3 oolumn 769V line 5l, for' "parts" read n part fi-if; .column 9, line/s' 39 and 40, for "autogsnous" read .autogeneous '-w.

Signed .and sealed this 10th day vof" May 1960.,

(SEAL.) Attest:

KARL H.. AXLINE ROBERT C. WATSON Attesting Olcer Commissioner of Patents 

1. THE METHOD OF PRODUCING FROM SUGAR CANE A SUBSTANTIALLY HOMOGENEOUS PULP CONSISTING PRINCIPALLY OF DIGESTED VASCULAR FIBRE COMPRISING SEPARATING FROM THE CRUSHED CANE SUBSTANTIALLY ITS ENTIRE CONTENT OF PARENCHYMATOUS TISSUE, WHILE LEAVING A RESIDUE COMPOSED OF A HETEROGENEOUS MIXTURE OF SUBSTANTIALLY THE ORIGINAL QUANTITIES OF FIBRO-VASCULAR BUNDLES AND SUBSTANTIALLY THE ORIGINAL QUANTITIES OF PERIPHERAL SCLERENCHYMA, FORMING THE SAID RESIDUE INTO A SUSPENSION IN LIQUID OF 5% TO 15% SOLIDS, DIGESTING THE SAID SUSPENSION UNDER PERDETERMINED ELEVATED TEMPERATURES WITH AN ALKALI COOK TO EFFECT PULPING OF SUBSTANTIALLY ALL THE FIBRES BELONGING TO THE FIBROVASCULAR BUNDLES WITHOUT SUBSTANTIAL DEGRADATION THEREOF, WHILE SOFTENING THE SCLERENCHYMA OF THE PERIPHERAL FIBRE BUNDLES, AND THEREAFTER REMOVING THE PERIPHERAL SCLERENCHYMA FROM THE PULPED VASCULAR CANAL FIBRES. 